No, when they break the mercury gets released, which is extremely toxic -- true. However, during regular usage they emit toxic gasses too. Phenols (sp?) and so on.
Yes, the efficiency is a hoax. There's no completed study that shows 80% savings. Mostly, the energy-intense production, and more importantly recycling (fun fact: the old ones that actually get returned by the users don't really get recycled in Germany, they just dump it like radio active waist in temporary storages). Moreover, they use lightning capacity over time (and quite noticeably, I think it's something like 80% less efficiency after 2k hours).